16-Watt FM Amplifier

This High-fidelity amplifier is designed for use with an FM tuner. Its power supply is capable of supplying plate and filament voltages for the tuner.

For simplicity and ease in construction, the amplifier consists of three stages: The first stage, using a 6J5 tube, is the phase inverter, needed for supplying two out-of-phase voltages to the succeeding push-  pull driver stage. This stage employs a 6C8-G dual triode and feeds the push-pull 6L6-G output tubes. About 16 watts of undistorted audio power is obtained here, more than enough for a good-sized home, but the reserve power is assurance of distortion-free operation. A 5X4-G is used as a rectifier, while a VR-105 voltage regulator tube is optional.

The entire unit is easily accommodated on a 7 x 11 x 2-inch chassis, even leaving some spare room in one corner. But this may come in handy later on, if it ever becomes desirable to install a high-gain input stage to increase the overall gain, so that low-level microphones can be used.

The power supply also is capable of powering an additional broadcast band AM tuner. A single six-pole three-position rotary switch serves to take care of switching the power to either tuner and also turns the a.c. supply on and off. Only the filament voltage is switched from tuner to tuner, since no plate current can be drawn by the tubes whosefilaments are not heated. If the constructor has no desire to use thi amplifier with any tuner other than the FM unit, he can omit thi; switch and the second power-output receptacle as well as one of the in put receptacles.

Although not shown in the diagram, the six sections of the rotar switch have been paralleled to give a three-pole, three-position switch oJ increased current-carrying capacity. Reference to the diagram will show that the switch is so wired that when it is in the center position, th( amplifier is turned off; when moved to either side of center the amplifiei is on, and filament voltage switched to one of the power receptacles Incidentally, the switch was mounted toward the rear of the chassis tc keep the a.c. leads away from the input tubes, necessitating the use of a mounting bracket and extension shaft.

The phase inverter

The phase inverter used is of the type in which the out-of-phase voltage is obtained from the cathode circuit. Actually, the 25,000-ohm resistor at the ground end of the 6J5 cathode is really part of the plate circuit. Where additional amplifier gain is not needed, this form of phase in¬verter is preferable (from the standpoint of simplicity) to the double-triode type, and was used for this reason.

Notice the 250,000-ohm resistors connected from the plates of the 6L6-G tubes to the plates of the 6C8-G tubes. These are the feedback resistors furnishing negative feedback which contributes so much to the excellent frequency response, and to the hum-free operation.

This method of feedback was purposely used, so that the constructor would have no trouble in getting the amplifier to operate properly the first time it is tried out. The polarity or magnitude of the feedback voltage is not dependent on the output transformer, so the constructor need not fear that the amplifier will squeal and howl like a banshee when first turned on. Incidentally, no output transformer was incorporated on the chassis, because the speaker used by the writer has the transformer built onto it.

List of Parts

1—600-ohm, 1/2-w resistor
1—3,000-ohm, 1/2-w resistor
2—25,000-ohm, 1/2-w resistors
1—50,000-ohm, 1/2-w resistor
4—500,000-ohm, 1/2 - w resistors
4—250,000-ohm, 1/2-w resistors
2—100,000-ohm,1/2-w resistors
1—150-ohm, 10w resistor
1—6,000-ohm, 10-w resistor
1—1-meg volume control
1—7 x 11 x 2-inch chassis
1—7 x 11-inch bottom plate
2—Octal steatite sockets
4—Octal sockets
2—5-prong sockets
1—5-prong plug
1—3-prong receptacle
1—3-prong plug
2—Input receptaeles
2—Input connectors
1—8-8-8-f, 450-v elect, cond.
3—.l-f, 400-v condensers
2—.05-f, 400-v condensers
2—10-f, 25-v elect, condensers
1—10-f, 50-v elect, condenser
1—4-f, 450-v elect, condenser
1—Power transformer, 675 v, 175 ma, c.t. 5 V, 3 amp; 6.3 v, 5 amp 1—175-ma, 20-h choke 1—75-ma, 30-h choke
1—Push-pull   plate   to   voice-coil   output transformer 5,000 ohms c.t. primary
1—6-pole,
3 position rotary sw.
2—6L6-G tubes
1—5X4-G tube
1—6C8-G tube
1—6J5 tube
1—VB-105 tube